Dynamical signature of a stellar bulge in a quasar-host galaxy at z  ≃  6

¹ Dipartimento di Fisica, Università di Trieste, Sezione di Astronomia, Via G.B. Tiepolo 11, 34131 Trieste, Italy
e-mail: roberta.tripodi@inaf.it
² INAF – Osservatorio Astronomico di Trieste, Via G. Tiepolo 11, 34143 Trieste, Italy
³ IFPU – Institute for Fundamental Physics of the Universe, Via Beirut 2, 34151 Trieste, Italy
⁴ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁵ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁶ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁷ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK

Received: 13 October 2022
Accepted: 17 January 2023

Abstract

We present a dynamical analysis of a quasar-host galaxy at z ≃ 6 (SDSS J2310+1855) using a high-resolution ALMA observation of the [CII] emission line. The observed rotation curve was fitted with mass models that considered the gravitational contribution of a thick gas disc, a thick star-forming stellar disc, and a central mass concentration, which is likely due to a combination of a spheroidal component (i.e. a stellar bulge) and a supermassive black hole (SMBH). The SMBH mass of 5 × 10⁹ M_⊙, previously measured using the CIV and MgII emission lines, is not sufficient to explain the high velocities in the central regions. Our dynamical model suggests the presence of a stellar bulge with a mass of M_bulge ∼ 10¹⁰ M_⊙ in this object, when the Universe was younger than 1 Gyr. To finally be located on the local M_SMBH − M_bulge relation, the bulge mass should increase by a factor of ∼40 from z = 6 to 0, while the SMBH mass should grow by a factor of 4 at most. This points towards asynchronous galaxy-BH co-evolution. Imaging with the JWST will allow us to validate this scenario.